update provider specs to stub at right layers

The provider specs where globally mocking the world to get around
having to stub `load_current_resource`. It's easier to just mock this
method for all cases *except* the few specs that test the
`load_current_resource` method.
RSpec #stub! is deprecated, use #stub

http://blog.firsthand.ca/2010/09/rspec-stub-vs-stub.html
Additional test coverage for [COOK-2353]

We had no unit test coverage for the core bit of functionality
introduced in [COOK-2353]. This new coverage ensures
`Chef::Provider::Service::Runit#run_action(:enable)` only makes an
internal call to `#restart_service` if the embedded run_script
template resource is updated AND the restart_on_update attribute is
true (which is default).
Merge pull request #22 from opscode-cookbooks/COOK-2353-additional-test-coverage

Additional test coverage for [COOK-2353]